1. Field of the Invention
The present invention relates to a ferrite carrier core material and a ferrite carrier used for a two-component electrophotographic developer used in copying machines, printers and the like, and an electrophotographic developer using the ferrite carrier.
2. Description of the Related Art
The method of electrophotographic development is a method in which toner particles in a developer are made to adhere on an electrostatic latent image formed on a photoreceptor to develop the image. The developer used in this method is classified into a two-component developer composed of a toner particle and a carrier particle, and a one-component developer using a toner particle alone.
As a development method using a two-component developer composed of a toner particle and a carrier particle among those developers, a cascade method and the like were formerly employed, but a magnetic brush method using a magnet roll is now in the mainstream.
In a two-component developer, a carrier particle is a carrier substance which is stirred with a toner particle in a development box filled with the developer to thereby impart a desired charge to the toner particle, and further transports the charged toner particle to a surface of a photoreceptor to thereby form a toner image on the photoreceptor. The carrier particle remaining on a development roll holding a magnet is again returned from the development roll to the development box, mixed and stirred with a fresh toner particle, and used repeatedly in a certain period.
In a two-component developer, unlike a one-component developer, a carrier particle has functions of being mixed and stirred with a toner particle to charge the toner particle and transporting the toner particle, and has good controllability on designing a developer. Therefore, the two-component developer is suitable for full-color development apparatuses requiring a high image quality, high-speed printing apparatuses requiring reliability and durability in image maintenance, and other apparatuses.
In a two-component developer thus used, it is needed that image characteristics, such as image density, fogging, white spots, gradation and resolving power, exhibit predetermined values from the initial stage, and additionally these characteristics do not vary and are stably maintained during endurance printing. In order to stably maintain these characteristics, properties of a carrier particle contained in a two-component developer need to be stable.
As a carrier particle forming a two-component developer, an iron powder carrier, such as an iron powder coated on the surface with an oxide film, or an iron powder coated on the surface with a resin, has conventionally been used. Since such an iron powder carrier has a high magnetization and a high electroconductivity as well, the carrier has an advantage of easily providing images well reproduced on the solid portion.
However, since such an iron powder carrier has a heavy true specific gravity of about 7.8 and a too high magnetization, stirring and mixing thereof with a toner particle in a development box is liable to generate fusion, so-called toner spent, of toner-constituting components onto the iron powder carrier surface. The generation of such toner spent decreases the effective carrier surface area, and is liable to reduce the triboelectric chargeability with the toner particle.
In a resin-coated iron powder carrier, the resin on the surface exfoliates due to stresses during endurance printing, and a core material (iron powder) having a high electroconductivity and a low dielectric breakdown voltage is thereby exposed, and leakage of the charge thereby occurs in some cases. Due to such leakage of the charge, an electrostatic latent image formed on a photoreceptor is broken, brush streaks and the like are generated on the solid portion, and a uniform image can hardly be obtained. For these reasons, iron powder carriers such as an oxide film-coated iron powder or a resin-coated iron powder have not been used recently.
In recent years, in place of an iron powder carrier, a ferrite having a light true specific gravity of about 5.0 and a low magnetization is sometimes used as a carrier, and a resin-coated ferrite carrier coated on the surface with a resin is often used, whereby the life of the developer has been remarkably prolonged.
The method for manufacturing such a ferrite carrier generally involves mixing ferrite carrier raw materials in predetermined amounts, and thereafter calcining, pulverizing and granulating and then sintering the mixed material; and depending on conditions, the calcination may be omitted.
Meanwhile, the environmental regulation has recently become strict, and the use of metals such as Ni, Cu and Zn comes to be avoided and the use of metals adapted to the environmental regulation is demanded; then, ferrite compositions used as a carrier core material have been shifted from Cu—Zn ferrites and Ni—Zn ferrites to manganese ferrites, Mn—Mg—Sr ferrites and the like, which use Mn.
Japanese Patent Laid-Open No. 08-22150 describes a ferrite carrier in which a part of a manganese-magnesium ferrite is replaced by SrO. It is contended that when the ferrite carrier is used as a developer with a toner, by reducing a variation in magnetization among ferrite carrier particles, the developer is excellent in image quality and durability, and friendly to the environment, and has a long life, and is excellent in environmental stability. However, the ferrite carrier described in Japanese Patent Laid-Open No. 08-22150 cannot satisfy simultaneously both a uniform surface property having a reasonable unevenness and a high charge imparting capability. If the sintering temperature is made high, since the surface property exhibits much of smooth portions and becomes nonuniform, not only the distributions of the resistance and the charge after resin coating are broadened, but also the strength to stirring stresses decreases. If the sintering temperature is made low, the surface apparently has a wrinkly uniform surface property, but since the value of the BET specific surface area becomes large, the charging property becomes low and the environmental difference becomes large.
Japanese Patent Laid-Open No. 2000-233930 discloses a carrier core composition containing manganese oxide and iron (III) oxide in certain proportions, containing titanium dioxide in a specific amount, and forming substantially a spinel phase material. The carrier core composition is contended to be environmentally safe and nonhazardous.
However, since the carrier core composition described in Japanese Patent Laid-Open No. 2000-233930 is a manganese ferrite, the composition has a low resistance, and a deterioration of image quality, such as the occurrence of fogging and a deterioration of gradation, are apprehended.
Carrier core materials using Mg are proposed as replacements of carrier core materials using Mn. For example, Japanese Patent Laid-Open No. 2010-39368 describes a carrier core material containing magnesium, titanium and iron in certain proportions and having a BET specific surface area in a specific range. It is contended that the carrier core material provides a desired resistance, a medium one or a high one, while exhibiting a high magnetization, and is excellent in the charging property, and has both of a surface property having a reasonable unevenness, and uniform shapes.
Since the carrier core material described in Japanese Patent Laid-Open No. 2010-39368 has low contents of manganese and titanium, the material basically exhibits properties of a magnetite, and since the magnetization of a low magnetic field side is low, the occurrence of carrier beads carry over in actual machine operations is apprehended.
In consideration of these conventional technologies, a ferrite carrier for an electrophotographic developer, which has a reasonable resistance and magnetization, and an excellent charging property, can maintain a high charge particularly at a high-temperature and high-humidity to thereby give a good environmental dependency, has been demanded.
Therefore, it is an object of the present invention to provide a ferrite carrier core material and a ferrite carrier for an electrophotographic developer, which have a reasonable resistance and magnetization, and an excellent charging property, and can maintain a high charge particularly at a high-temperature and high-humidity to thereby give a good environmental dependency, and an electrophotographic developer using the ferrite carrier.